Hank: Sat you’re absent-mindedly spooning yogurt into your mouth, when you happen to glance at the container and notice that it says something a little weird: “Contains live and active cultures.” There are bacteria! Inside your yogurt! And for some reason the company is advertising this!?

The fact that your yogurt has live bacteria inside of it might sound kinda gross. But even though some species can be dangerous, lots of bacteria -- including the ones in yogurt -- are not bad for you. In fact, they’re responsible for turning milk into that yogurt that you’re eating. To make a thick, creamy yogurt, you need to start with some milk. Milk is just a mixture of different fats, proteins, and this sugar, called lactose, all suspended in water. If you want yogurt, you need to find ways to thicken this milk to get that nice creamy, scoopable texture, but you don’t want to separate the milk into solid chunky curds and liquid whey, otherwise you’d be making cheese.

So, a first step in most modern yogurt-making is to heat up the milk a lot. Not only does this kill off any mystery microbes that might be hanging around, but it also starts evaporating some of the water, which helps thicken the milk. More importantly, the high temperatures start breaking down some of the whey proteins inside, called beta-lactoglobulin, which changes their shape These unfolded protein structures then start interacting with other clusters of proteins, called caseins. And this interaction is key to clumping the milk proteins just enough to make yogurt, but not chunky curds.

Then, you cool down the milk a little bit, and add some heat-loving, or thermophilic, bacteria. Bacteria can eat the lactose sugars in the milk and produce lactic acid through a process called fermentation. When these bacteria ferment the milk and make it acidic enough, the structure of those casein proteins starts to change. The proteins form a new, mixed, mesh-like structure that creates a thick, creamy texture. Plus, all that lactic acid helps give your yogurt its characteristic tangy flavor.

Certain countries standardize what kind of bacteria you need to use to make mass-produced yogurt. In the US, for example, the FDA requires people to ferment their dairy with two particular species of bacteria, Lactobacillus bulgaricus and Streptococcus thermophilus. That last species is in the same genus as the bacteria that can cause strep throat, and a lot of other infections. But it’s totally harmless.

Within the two species of bacteria used for making yogurt, there are different strains of bacteria, kind of like there are different dog breeds. And different strains of bacteria might produce different flavors of yogurt, making it more sour or bitter or tangy or creamy. Then, you can add other things to your yogurt, like sugars, colors, and artificial flavors, or even other bacteria.

For example, there are some species of bacteria that we consider “probiotics,” because they may help our gut microbiome and overall health, although there’s not a lot of concrete evidence to support this right now. Different types of yogurt, like greek yogurt, for example, come from other modifications of this cooking process, like using different temperatures or fermentation times.

So the next time you open a cup of yogurt for breakfast, don’t forget to thank the bacterial friends you’re eating for making that yogurt delicious.

So thanks to them, and thank you for watching this episode of SciShow, brought to you by our patrons on Patreon, who are the ones we should really thank, even more than our yogurt bacteria, because they make this show possible. If you want to help us out, just go to patreon.com/scishow. And if you want to keep getting smarter with us, you can go to youtube.com/scishow and subscribe!